Surely our brain (rather than heart beat or anything else) is what makes us alive. Brain activity in a foetus starts at 6 weeks. However, that doesn't mean that is any high level intelligence at that stage, or even consciousness at all. It may be that a 6-week foetus has the amount of brain activity of an insect and therefore perhaps a similar moral worth (although this is highly speculative). Read here for how the brain develops during pregnancy:
https://www.nytimes.com/2005/06/19/books/chapters/the-ethical-brain.html (The author of the article is Michael S. Gazzaniga. I googled his name and I assume he is the one for whom I find a wikipedia article "professor of psychology at the University of California, Santa Barbara, where he heads the new SAGE Center for the Study of the Mind. He is one of the leading researchers in cognitive neuroscience, the study of the neural basis of mind.")
Quoting from above article: "not until the end of week 5 and into week 6 (usually around forty to forty-three days) does the first electrical brain activity begin to occur. This activity, however, is not coherent activity of the kind that underlies human consciousness, or even the coherent activity seen in a shrimp's nervous system. Just as neural activity is present in clinically brain-dead patients, early neural activity consists of unorganized neuron firing of a primitive kind. Neuronal activity by itself does not represent integrated behavior." (quoted from above).
At 13 weeks, the article claims that "the fetus is not a sentient, self-aware organism at this point; it is more like a sea slug, a writhing, reflex-bound hunk of sensory-motor processes that does not respond to anything in a directed, purposeful way."
The article further claims that at week 23 the foetus can can "respond to aversive stimuli" but that prior to that date it is not "viable".
The article further claims that if an adult had suffered massive brain damage, reducing their brain to the level of development of a 23-week foetus, the patient "would be considered brain dead and a candidate for organ donation".
According to Tomás Ryan, (assistant professor of neuroscience at Trinity College Dublin), quoted in
https://www.irishtimes.com/opinion/...ther-scientific-answers-on-abortion-1.3506968 :
"Crucially, the co-ordinated brain activity required for consciousness does not occur until 24-25 weeks of pregnancy. We cannot say when consciousness first emerges, but it cannot rationally be called before the end of the second trimester at 24 weeks of pregnancy." He also adds: "The thalamus (necessary for pain and conscious perception) does not appear until the end of the second trimester."
To all of the above, however, I think we should add that it is morally wrong to carry out an action that has say a 50% chance of causing moral bad, a 50% chance of causing nothing, and NO chance of causing moral good. To illustrate this point, if I gave you a gun with 6 chambers and 3 bullets and a dog, it would be morally wrong of you to point the gun at the dog's head and pull the trigger once, and couldn't be defended on the basis that there is a 50/50 chance that no bad will come of it.
Applying this logic to the abortion debate, if there is a foetus that has a 50/50 chance of being alive, conscious and able to feel pain, and a 50/50 chance of being entirely unconsciousness, not yet alive or able to feel pain, then it would be morally bad to carry out an abortion in such a case, all other things being equal, e.g say in a case where it would cause no problem to the woman or anyone else to have the baby, and would not make her life worse. Say she wanted a baby anyway, and the only reason to have an abortion was to delay for a year so she could save up more money first. So I think the benefit of the doubt should be applied, and I also think any vegan that would follow the benefit of the doubt principle in order to explain why they don't eat certain types of seafood would be caught in rather a contradiction if they wouldn't apply that same logic to human life.